There's the Doctor, of course. We can all agree on that. Even in his least impressive incarnations (sorry, Peter), this time traveller from another world might possibly be the best, most fascinating character ever invented.

Then there are the locations. Backward in time, forward in time, on Earth or on a distant planet at the end of the universe: it's all grist to the mill. In Doctor Who, there's nowhere you can't go and there's nothing you can't see.

And there are also the monsters. But we'll have to agree to disagree on that one.

We don't know about you, but we're mad about Doctor Who for all of that (except most of the monsters, obviously). But there's something else. There's also the way that, from time to time, a kids' TV series casually takes a swerve into some of the deepest issues imaginable. It's so Doctor Who, and we love it. It's also why we think The Impossible Planet/The Satan Pit is some of the best Doctor Who we've ever seen.

The beginning of The Impossible Planet doesn't give many clues that this is the way things are going to go. While extremely well done, it's a classic base under siege story. It doesn't even seem like a new pattern in that theadbare weave: the resonance with Robots Of Death, and to a lesser extent Frontios, is impossible to miss. Nevertheless, while the ingredients are familiar, they're also of very high quality. Direction is great without being flashy, and aided by claustrophobic sets and shaking-camera scenes where the ship for once actually does a good impression of being flung around, The Impossible Planet is an impeccably taut horror thriller.

We were expecting more of the same in The Satan Pit: rampaging Ood, the crew being picked off one by one, the Doctor saving the day, you know the drill. And sure, they sort of did that. The thing is, though, that they didn't stop there. What starts as a standard if well-executed horror story opens up into a meditation on the origins of evil that shakes the Doctor's certainty about the universe to the core. Now that's impressive.

The horror doesn't stop in The Satan Pit, of course. And while it's crammed with cliches involving ventilation shafts and riddled with plot and science blunders, nevertheless it works like a charm. The ending, with Rose and the others seconds away from death in the rocket, goes beyond the kind of scariness you get with monsters: it's so real that it's viscerally terrifying. But what really lifts The Satan Pit to another level is the story it's intercut with.

Compared to the frenetic scrambling going on above, the Doctor's story's pretty quiet. And actually, that's partly what makes it so effective. The Doctor's discussions with Ida, especially after the cable breaks, are all the more effective for their calm. And the sheer understatedness of the Doctor simply letting go and falling into the void is exactly what gives it its wow factor. It's living proof that action doesn't have to be, well, actiony to have an impact.

The Doctor's confrontation with the Beast is probably the strongest scene in here, simply because of the way it plays against convention. The way you're supposed to do this stuff is to have the Doctor chitchatting with Powerful Evildoer, all casual-like, until the Doctor whips out his clever plan and reigns victorious. But that's not what we see here. The Doctor's chitchatting is with himself, because the Beast is busy joyriding. The Doctor's monologue is a complete tour de force and David Tennant carries it off effortlessly, ranging through the Doctor's emotions with perfect naturalness. Most enjoyable is the highly unusual sight of the Doctor deceiving himself, choosing not to understand the nature of the Beast. His confidence in his grasp of the universe is normally so complete that it's one of his defining characteristics: to see how he reacts when that's challenged gives us a fascinating insight into his character. And we love the way his descent into darkness symbolises his leaving behind his certainty. Good stuff, eh?

Is there a religious explanation for the Beast? Is he the Devil? Is he, come to that, Sutekh? Despite all the sixes, not to mention the presence of the remarkable Gabriel Woolf, we don't see any reason to think so. We think it's made pretty clear that the Beast is no one incarnation of evil - after all, it claims to be all of them. In some mysterious way, possibly by rampaging through the universe shedding evil molecules before it was caught, it's been the seed of evil but not the flowering. That's if you believe it's in all evil, of course, and that's not necessarily a given. We're willing to bet that something claiming to be evil incarnate is probably a pretty good liar.

And the fact that it claims to be pre-universe? Does that imply that the universe must have been created? Nope. We're puzzled by the Doctor's trouble with this point. He says he has no problem with the idea of something beyond the universe, but that something before the universe is impossible: if we're getting our physics right, these are pretty much the same thing. Left, right, up, down, before, after - they're all just different directions as far as the universe is concerned.

Start out with horror, end up with ruminations on the nature of the universe. (And horror.) That's Doctor Who at its best.

And it's not just the philosophising that makes these eps so good. First, there are some genuinely good monsters. The Beast looks absolutely incredible, and on a TV budget too, and put-upon monsters-by-force the Ood are utterly (and creepily) convincing: light years from the rubber nightmares of yore. Even their killer lightbulbs, which if badly done would have looked hilarious, are positively menacing. The Ood dudes themselves with their willing-slave status (ick) are just different enough from the Vocs in Robots Of Death to be interesting, and the crew's attittudes to them, from dismissive to just slightly contemptuous, are thought-provoking. And the possessed Toby's skin scribbles and red eyes are a simple idea but a gobsmackingly effective one.

As for the characters, both regular and guest casts do a bang-up job. The crew comprise one of the strongest supporting casts we've ever seen in Doctor Who: there isn't one who's less than fantastic. Shaun Parkes was terrific as David Tennant's sidekick Rocco in Casanova, and here he's superlative: he brings a down-to-earth quality to the role of acting captain that makes him completely believable. Head of Security tends to be a role reserved for gun brandishing and general macho posturing, but Danny Webb manages to be just the right kind of tough without once foaming rabidly at the mouth. Claire Rushbrook is a wonderfully thoughtful companion for the Doctor in their journey down t'pit. Ronny Jhutti is scared without being cowardly as the Ood wrangler, and MyAnna Buring as the ill-fated Scooti is perfect as the friendly junior officer.

But what about Toby? Why haven't we mentioned him? Well, it's because Will Thorp is such a knockout that he deserves a paragraph to himself. He's utterly convincing as the quivering academic, and he hits the evil possessed Beast personification out of the park. We don't think there's been a single scarier scene in Doctor Who than that shot of Toby standing impossibly on the surface with an evil grin and without a spacesuit: if that doesn't send a chill down your spine, then you must be an invertebrate. And the shot of him shushing the Ood is terrifying too. Even more impressive is the fact that in the rocket he manages to rant evilly without once being the slightest bit gigglesome, and that's a task which has crushed strong men to powder. Go him.

Then there's the Doctor and Rose - and it's important to bracket the two of them together, because there's a hell of a lot in here about their relationship. And it's not all as it looks on the surface either. With all the chatter about mortgages and the Doctor believing in Rose, it would be easy to mistake their relationship here for the goopy in-loveness that the writing has sometimes hinted at. But you only have to listen carefully to what they really say to figure out that that's not what's going on at all. It's obvious how Rose feels about the Doctor: all her efforts to get the crew to pull together and rescue themselves are to one end - so that they can then rescue the Doctor. And she doesn't want to live without him, either.

The Doctor's feelings, though, are a lot more complex. When he thinks the TARDIS is gone, he says "I need my ship! It's all I've got! Literally the only thing!" Clearly, he isn't counting Rose in the one-item list of things that are important to him. Then in the conversation about settling down, Rose brings up the idea of them moving in together, but the Doctor's absolutely unresponsive: all his conversation is about how weird it would be for him to live in a house. Rose's fussing about the Doctor's safety when he decides to go down the cable obviously irritates the Doctor a little, and he responds to her gushing her relief that he made it OK pretty much by telling her to pull herself together. It's important to note, too, that Rose is very unhappy that the Doctor's going to take the plunge down the mineshaft, but it doesn't cross his mind for a second to not go for her sake. With the TARDIS gone, the Doctor is all Rose has, but that still doesn't stop him.

As for the Doctor deciding that destroying the Beast is the right thing to do because of his belief in Rose, that isn't some moony-eyed declaration: it just says that he believes she is sufficiently resourceful to get herself out of the danger he's about to put her in. Yes, "Tell her...oh, she knows" is the last thing he says before dropping into the pit: there's no doubt she's important to him. We don't even think there's much doubt that he loves her. But he loves her in his own way, not in the way she loves him and not, we believe, in an entirely human way either. Both Billie Piper and David Tennant manage all this complexity, and the deep themes the episode touches on, superbly.

And as well as all this, both the Doctor and Rose are realised in a way thatís gratifyingly true to both of them. "Moon base, sea base, space base - they build these things out of kits." Unlike that ghastly line in Idiot's Lantern about the three complicated and brilliant reasons why somebody should listen to him, this is good Doctor flippancy. His manicness is reined in and we get to see more than we often do of the thoughtfulness underneath. And Rose, from her Doctor-oriented bravery to her wisecracks, is also the character at her most on-key.

What else works? The CGI black hole looks amazing, and the disappearance of the Scarlet System lends an awesome sense of proportion. The "don't turn around" scene shows just how simple it is to scare the bejesus out of an audience (provided you have a brilliant actor and a brilliant voice artist, that is). The script's often hilarious ("We haven't got a swimming pool either. Or a Tesco's.") and luckily, Billie Piper's comedy skills are improving: her delivery of "Protein one with just a dash of three" is comedy gold. All the effort filming MyAnna Buring underwater paid off, as although we don't see her for long, her slowly revolving corpse looks, er, lovely. Ta-daa moment when they find the TARDIS aside, even Murray Gold's music isn't as obtrusive as it usually is. Although if we have to hear that look-out-touching-moment Woo-oo-oo-oo theme again we swear we're going to stab somebody.

It can't be perfect, though, surely? Of course not. There are lots of plot and science howlers, for a start (which we gleefully rip apart in the Outtakes section). The disappearance of the TARDIS is being well overused this series, in a Boy Who Cried Bad Wolf kinda way. The device of bringing in a crew member we've never set eyes on for the sole purpose of Ooding them to death takes "red-shirt extra" to truly absurd lengths. The crew move on a little too quickly from all the deaths, skipping straight to the elegy part and missing out the shock horror part pretty much altogether. And the fate of the poor Ood is a bit rushed too: not only do none of the crew in their scramble to get away spare them a thought, we get the feeling that when the Doctor says that he wasn't able to save them, the crew is scratching their heads and thinking "Who?". But these are minor quibbles in anyone's book. Without a shadow of a doubt, this is 21-carat gold.

MORAL: Get thee behind me, Satan.

OUTTAKES

ORBITUARY

It's impossible to orbit a black hole? Rubbish. Presumably, though, they didn't want to get all tangled up in an explanation about event horizons.

CíMERE, YOU

The Doctor has now officially yapped on about the gloriousness of humans one too many times. That hug is the icing on a stale and sickly cake. (The hug he gives Rose, on the other hand, is perfectly judged.)

MUSICAL CHAIRS

If there are no seats for the Doctor and Rose, where was the captain that was lost on the voyage planning to sit? Or the red-shirt extras, come to that?

CAN YOU HEAR ME, MOTHER?

At last, they've actually gone somewhere where Rose can't phone home. And we love the way Rose throws the phone away when the Beast gives her a bell.

INFLATABLE

What's going on with the gravity on this planet? Obviously there is some, because the mining Oods, the Doctor and Ida don't seem to have any trouble keeping their feet on the floor. So why is Scooti's body floating around in space?

EVA

How does the computer pick up Scooti's chip when she's floating around outside?

SNAP, CRACKLE, POP

Urgh. Does the Doctor have to say "This is going to be the best Christmas Walford's ever had"? What's wrong with "We must act now!"? Or, possibly, "It's a million to one chance, but it might just work"?

GSOH

They just couldn't resist the evil chuckle, could they?

AN AIR OF MYSTERY

The Beast's dramatic revelations (hur hur) about the crew members are all very nice, but they don't go anywhere. We didn't have to find out exactly what Jefferson's wife never forgave him for or anything, but a bit more payoff would have been a lot more effective.

SO VERY LONELY

So the Beast will arise from the pit and do battle with God, will it? Would that be a lonely god, by any chance? Sorry, guys, but you've lost us with this line of thinking: the Doctor is many things, but nothing is going to convince us he's a god.

NUKE HIM

We love the fact that Rose's enlightened and compassionate saving of Toby is, for once, precisely the wrong thing to do.

WINK WINK

'"Ventilation shafts?" " I appreciate the reference..."' Well, we didn't. As we point out with tiresome regularity, acknowledging a cliche doesn't make it any less of one. Having said that, we freely admit that the tunnel sequence, incorporating tension, heroic self-sacrifice and arse humour, is very well done. Although...

BREATHING ROOM

... what's all the blather about switching the oxygen on when there are grilles in the tunnels which connect with the oxygenated bits?

ITíS LIKE TALKING TO MYSELF

Although the moment when Toby turns to the Ood and put his finger to his lips is supremely creepy, why is it actually necessary? The Beast's controlling both of them.

I DREAMT I WAS LANDING ON BILLOWY BILLOWS OF SOFT SILK AND SATIN MARSHMALLOW-STUFFED PILLOWS

An air cushion? If it manages to break a lengthy fall, it must be more like an air bouncy castle. Ordinary human-breathable air, after all, doesn't cushion much at all: we know, because we've fallen off things in the interest of science. And how did the mysterious powers that be work out what the Doctor breathes, anyway?

BROKEN MUFFLER

We know there's a tube to the surface, but still: could the Doctor really hear the rocket leaving from more than twenty miles underground?

WEíLL NEVER HAVE TO SEE THOSE VASES YOUR MOTHER GAVE US AGAIN

If the mysterious powers that be (TM) thought the Beast was so dangerous it was worth trapping with a couple of pots next to a black hole, why didn't they just fling it into the black hole in the first place?

IíM NOT INHABITING THAT RABBLE

When Rose undoes Tobyís seatbelt, why doesnít the Beast just jump out of Toby and into one of the other crew members?